The present invention relates to a process for the manufacture of a component made of opaque synthetic quartz glass. Moreover, the invention relates to a quartz glass tube manufactured in accordance with said method.
Quartz glass tubes, rods, panels, and blocks, either as semi-finished or finished goods, are important components for heat engineering applications, in which good thermal insulation along with high temperature stability and thermal fatigue resistance are essential. Applications of the semi-conductor industry put ever increasing demands on the purity of the opaque quartz glass tubes and components used. Reactors, diffusion tubes, thermal shields, cones or flanges are but a few examples. These applications require the glass components to be opaque mainly in the infrared region of the spectrum. The impurities present in low-purity quartz glass contribute to the opacity of the glass. However, quartz glass manufactured from pure starting materials is transparent, and must be made opaque by artificially introducing pores into the glass.
In this context, the manufacture of thin-walled opaque quartz glass tubes or tube sections by reshaping an opaque blank in a thermal reshaping process poses a formidable problem because of the low wall thickness such tubes or tube sections have, which causes these parts to easily become transparent while heated in the reshaping process, especially if highly pure starting materials are used for manufacture. This invention relates to the manufacture of components, above all of thin-walled tubes, of opaque quartz glass manufactured from pure starting materials.
A manufacturing method for opaque quartz glass from pure starting materials is described in EP-A1 816 297, proposing to create opacity in the quartz glass by preparing a powder mixture consisting of synthetic SiO2 particles with a mean particle size of 300 xcexcm and an additive in the form of silicon nitride powder, and melting the mixture. Upon melting, thermal decomposition of the Si3N4 powder releases the gaseous components of the powder mixture, such as nitrogen. The gaseous components generate pores in the softened quartz glass, and provide for the desired opacity of the form body. The form body is manufactured by placing the powder mixture in a graphite mold lined with graphitic felt and heating in a vacuum at a temperature of 1,800xc2x0 C. in an electrically heated furnace. Upon melting, the front of the softening and melting quartz glass migrates from the mold wall radially to the core producing the so-called xe2x80x9cmelting frontxe2x80x9d.
Any contaminations present can cause de-vitrification of the quartz glass, resulting in brittleness and reduced thermal fatigue resistance. Residual additive can also negatively affect these quality properties of the quartz glass. An inhomogeneous pore distribution is also detrimental. Vitrification may be accompanied by a pore growth process, in which larger pores grow to the disadvantage of smaller ones. Large pores, however, contribute only little to opacity, cause the density of the opaque quartz glass to be low, and reduce the mechanical stability and the serviceable life of the quartz glass form body.
The manufacture of quartz glass tubes from such form bodies is both work- and time-intensive, especially if high dimensional accuracy is required. High dimensional accuracy of the wall thickness is a general prerequisite in all applications, in which another component is attached to a quartz glass tube by melting.
It is an object of the invention to provide a method for the inexpensive manufacture of components, above all thin-walled tubes or tube sections, made of opaque quartz glass and characterized by high chemical purity and high dimensional accuracy.
It is also an object of the invention to provide a quartz glass tube manufactured by said method, especially for use in the production of semi-conductors.
The manufacture of the desired opaque quartz glass components is by a process comprising the following steps:
(a) Providing a starting material in the form of a granulated material of highly pure synthetic SiO2 comprised of at least partially porous agglomerates of SiO2 primary particles, with a compacted bulk density of no less than 0.8 g/cm3;
(b) Filling a mold with the granulate material and fabrication of an opaque quartz glass preform through a melting process;
(c) Reshaping the preform in a thermal reshaping process to form the opaque quartz glass component.
The process according to the invention entails at least two processing steps at high temperatures (hereinafter referred to as xe2x80x9cheat treatment stepxe2x80x9d or xe2x80x9cheat reshaping processxe2x80x9d). In steps (b) and (c) of the process above, the starting material and the preform made from the starting material, respectively, are subjected to processing at high temperatures. The process according to the invention is characterized by the fact that an opaque component made of pure quartz glass is obtained after the heat treatment steps described above, even from highly pure starting materials. The second heat reshaping process offers a low-cost opportunity to adjust the final dimensions of the opaque component to the desired values at high dimensional accuracy. This relates mainly to the wall thickness, inner and outer diameters of tube-shaped components, and the outer diameter of rod-shaped components.
It is an essential prerequisite of the process according to the invention to use in process step (a) above a starting material in the form of a granulated material made from highly pure synthetic SiO2. In a suitable highly pure SiO2 starting material for the purpose of the present invention the total content of contaminants, such as Li, Na, K, Mg, Ca, Fe, Cu, Cr, Mn, Ti, and Zr, is below 1 weight-ppm. In this context, doping agents are not considered contaminants.
The granulated material consists of at least partially porous agglomerates of SiO2 primary particles and has a compacted bulk density of no less than 0.8 g/cm3. Primary particles of this type can be fabricated by flame hydrolysis or oxidation of silicon compounds, hydrolysis of organic silicon compounds in accordance with the so-called sol-gel process or hydrolysis of inorganic silicon compounds in a liquid medium. Although primary particles fabricated by one of these methods are characterized by high purity, they are difficult to handle due to their low bulk density. Thus, it is common to compact this material by means of granulation procedures. Granulation causes the fine primary particles to form agglomerates of larger diameter. For the success of the process according to the invention it is essential that gases are trapped in the material when the granulated material is melted, which requires that there is a certain degree of porosity, which can be conferred either by open or closed pore spaces inside the individual agglomerates. In the preform melting process, the majority of the existing pore spaces close during the sintering and collapsing steps. However, previously open pore channels are converted into a multitude of fine closed pores capable of back-scattering IR radiation and, thus, conveying high IR opacity also. The required opacity can also be introduced into the quartz glass by using a granulated material consisting of agglomerates with a rugged surface structure showing superficial fissures. In the melting process, these fissures form a pore space that can entrap gases and, thus, form closed pores in the preform. These fine, closed pores scatter incident light which renders the preform opaque.
As a consequence, it is not necessary to add an additive that becomes volatile during vitrification in order to generate opacityxe2x80x94such as is done in the known process described abovexe2x80x94and consequently there is no risk of introducing contaminants into the quartz glass with an additive.
At a compacted bulk density of no less than 0.8 g/cm3, the SiO2 granulated material can be placed in the mold to produce a preform; the compacted bulk density is a measure of the porosity of the granulated material and can be determined in accordance with DIN ISO 787 part 11.
The invention is illustrated in the following on the basis of embodiments and the drawings. The drawings show in diagrammatic view: